Hugh Longbourne Callendar

Prior to Callendar’s work in the area of thermometry, there lacked an instrument for accurate and reliable temperature measurement. The gas thermometer was the standard for temperature scale at the time. These devices had significant limitations regarding their reliability and practicality, as they were costly and large.[10] The mercury thermometer was also used for temperature measurement, although it had a restricted range and was often too fragile for use.[10]

At Cavendish Laboratory, Thomson advised Callendar to study metallic resistance thermometry.[4] Werner von Siemens was the first to propose the use of a platinum resistance temperature detector in 1860, although his instrument readings were unstable.[2] Callendar developed an equation for the resistance of metal as a function of temperature, which was accurate to within 1% from 0-600°C.[2] This equation was used to develop a standard scale of temperature, which was later accepted by the Committee on Electrical Standards in 1899 to be used internationally.[2][11] By 1886 he had developed a design for an accurate platinum resistance thermometer, correcting the errors made by Siemens.[4] The thermometer measured temperatures with accuracy from  -190°C to 660°C. His results were praised by J.J Thomson for providing a new tool that “could determine temperatures with an ease and accuracy never obtainable before”.[6] Callendar improved the heat range of his platinum thermometer from -200°C up to 1000°C.[4][10] His design underwent vigorous testing at the National Physical Laboratory, which yielded confirmation of the reliability of the thermometer.[10]

Callendar’s platinum thermometer could be used to measure the melting point of metals, allowing it to be used in metal alloying. The apparatus was produced commercially by the Cambridge Instrument Company.[4] The production of modern platinum thermometers were based on Callendar’s model with the accuracy continually improved on, such as more purified platinum and making them smaller.[11] His work on the platinum resistance thermometer gave way for the development of the rolling-chart thermometer, which permits the measurement of climatic temperature over time.[1]

Although there are many other methods for temperature measurement, platinum resistance thermometers continue to be used. They are primarily used as calibrating devices due to their high levels of accuracy and stability over time.[12]

Illustration of calorimeter by H.L. Callendar

Callendar left Cambridge and spent two years working as a professor of physics at Royal Holloway College from 1891-1893.[1] In 1893 he commenced at McGill University in Montreal, Canada and was assigned to the second Macdonald Chair in Physics.[1] Leslie, Cecil and Guy were born during this time at McGill. While at McGill, Callendar conducted research on two topics in the area of thermodynamics. The first was research on steam engines alongside Professor of Mechanical Engineering John Thomas Nicolson.[1] The second was the study of electrical and thermal units using calorimetry techniques with Howard Turner Barnes.[1]

He also studied and published papers on the heat of gases. He calculated a boiling point of sulfur, which was four degrees below the existing accepted value.[2] The boiling point of sulfur on the International Temperature Scale in 1927 and Callendar’s value only differed by 0.07°C.[2]

Callendar developed a formula for the heat of steam, expressing it as a function of pressure and temperature, publishing it in his first paper on thermodynamics in 1900.[1] He was able to obtain experimental results consistent with his formula on multiple thermodynamic properties.[1] From this he was able to develop tables on the thermodynamic properties of steam for calculations in science and engineering. This was an important contribution, as there lacked a standard accepted set of steam tables. His Callendar Steam Tables were published (1915, 1922, 1927) and also published The Properties of Steam and Thermodynamic Theory of Turbines in 1920.[5][7] Turbine manufacturers accepted and used Callendar’s steam tables to invent steam machinery.[2]

Callendar and his son participated in the first International Steam Conference was held in 1929, which aimed to resolve inconsistencies between steam tables used globally.[1] After Callendar’s death the following year, his son Guy Stewart continued to work on steam properties. He published several works on the subject alongside professor Alfred Charles Glyn Egerton.[2]

In 1902, Callendar and Barnes developed the continuous-flow calorimeter, used for measurement of the heat capacity properties of liquids.[2] They used their apparatus to research the comparison of electrical and thermal units, calculating a value for one calorie that compares very closely to the currently accepted value.[2] Callendar’s continuous-flow calorimeter was further improved on and later used in laboratories for scientific experiments and research.[1] Their apparatus was used widely for the determination of the heat capacity of liquids and gas, as well as study of combustion gases.[13]

In 1896, Callendar was the first in Canada to perform X-ray experiments. The method produced satisfactory results that could be used for imaging in hospitals.[4]

Callendar also did work on fuels later in his career. He contributed to a paper on detonation of fuels, also including nuclear self-ignition, published in 1926.[2] In 1927 he published a further paper on detonation of paraffin fuels.[2]

During World War 1 Callendar was a consultant to the Board of Invention and Research (BIR). He worked mostly with the body responsible in charge of detecting and destroying submarines.[1]

He was also a member of the Air Inventions Committee (AIC), alongside his son Guy Stewart. Here they performed experiments on equipment and aircraft engines using radiography, locating defects and fractures.[1]